From f5ad5083404bb56c9de777dccb68c6672ef6487e Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Fri, 2 Dec 2022 17:27:24 +0100 Subject: mm: do not BUG_ON missing brk mapping, because userspace can unmap it The following program will trigger the BUG_ON that this patch removes, because the user can munmap() mm->brk: #include #include #include #include static void *brk_now(void) { return (void *)syscall(SYS_brk, 0); } static void brk_set(void *b) { assert(syscall(SYS_brk, b) != -1); } int main(int argc, char *argv[]) { void *b = brk_now(); brk_set(b + 4096); assert(munmap(b - 4096, 4096 * 2) == 0); brk_set(b); return 0; } Compile that with musl, since glibc actually uses brk(), and then execute it, and it'll hit this splat: kernel BUG at mm/mmap.c:229! invalid opcode: 0000 [#1] PREEMPT SMP CPU: 12 PID: 1379 Comm: a.out Tainted: G S U 6.1.0-rc7+ #419 RIP: 0010:__do_sys_brk+0x2fc/0x340 Code: 00 00 4c 89 ef e8 04 d3 fe ff eb 9a be 01 00 00 00 4c 89 ff e8 35 e0 fe ff e9 6e ff ff ff 4d 89 a7 20> RSP: 0018:ffff888140bc7eb0 EFLAGS: 00010246 RAX: 0000000000000000 RBX: 00000000007e7000 RCX: ffff8881020fe000 RDX: ffff8881020fe001 RSI: ffff8881955c9b00 RDI: ffff8881955c9b08 RBP: 0000000000000000 R08: ffff8881955c9b00 R09: 00007ffc77844000 R10: 0000000000000000 R11: 0000000000000001 R12: 00000000007e8000 R13: 00000000007e8000 R14: 00000000007e7000 R15: ffff8881020fe000 FS: 0000000000604298(0000) GS:ffff88901f700000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000603fe0 CR3: 000000015ba9a005 CR4: 0000000000770ee0 PKRU: 55555554 Call Trace: do_syscall_64+0x2b/0x50 entry_SYSCALL_64_after_hwframe+0x46/0xb0 RIP: 0033:0x400678 Code: 10 4c 8d 41 08 4c 89 44 24 10 4c 8b 01 8b 4c 24 08 83 f9 2f 77 0a 4c 8d 4c 24 20 4c 01 c9 eb 05 48 8b> RSP: 002b:00007ffc77863890 EFLAGS: 00000212 ORIG_RAX: 000000000000000c RAX: ffffffffffffffda RBX: 000000000040031b RCX: 0000000000400678 RDX: 00000000004006a1 RSI: 00000000007e6000 RDI: 00000000007e7000 RBP: 00007ffc77863900 R08: 0000000000000000 R09: 00000000007e6000 R10: 00007ffc77863930 R11: 0000000000000212 R12: 00007ffc77863978 R13: 00007ffc77863988 R14: 0000000000000000 R15: 0000000000000000 Instead, just return the old brk value if the original mapping has been removed. [akpm@linux-foundation.org: fix changelog, per Liam] Link: https://lkml.kernel.org/r/20221202162724.2009-1-Jason@zx2c4.com Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()") Signed-off-by: Jason A. Donenfeld Acked-by: Vlastimil Babka Reviewed-by: Liam R. Howlett Reviewed-by: SeongJae Park Cc: Yu Zhao Cc: Catalin Marinas Cc: David Hildenbrand Cc: David Howells Cc: Davidlohr Bueso Cc: Matthew Wilcox Cc: Sven Schnelle Cc: Will Deacon Cc: Jann Horn Signed-off-by: Andrew Morton --- mm/mmap.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) (limited to 'mm/mmap.c') diff --git a/mm/mmap.c b/mm/mmap.c index 74a84eb33b90..0024e615d069 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -226,8 +226,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk) /* Search one past newbrk */ mas_set(&mas, newbrk); brkvma = mas_find(&mas, oldbrk); - BUG_ON(brkvma == NULL); - if (brkvma->vm_start >= oldbrk) + if (!brkvma || brkvma->vm_start >= oldbrk) goto out; /* mapping intersects with an existing non-brk vma. */ /* * mm->brk must be protected by write mmap_lock. -- cgit v1.2.3 From 6c28ca6485ddd7c5da171e479e3ebfbe661efc4d Mon Sep 17 00:00:00 2001 From: Liam Howlett Date: Mon, 5 Dec 2022 19:23:17 +0000 Subject: mmap: fix do_brk_flags() modifying obviously incorrect VMAs Add more sanity checks to the VMA that do_brk_flags() will expand. Ensure the VMA matches basic merge requirements within the function before calling can_vma_merge_after(). Drop the duplicate checks from vm_brk_flags() since they will be enforced later. The old code would expand file VMAs on brk(), which is functionally wrong and also dangerous in terms of locking because the brk() path isn't designed for file VMAs and therefore doesn't lock the file mapping. Checking can_vma_merge_after() ensures that new anonymous VMAs can't be merged into file VMAs. See https://lore.kernel.org/linux-mm/CAG48ez1tJZTOjS_FjRZhvtDA-STFmdw8PEizPDwMGFd_ui0Nrw@mail.gmail.com/ Link: https://lkml.kernel.org/r/20221205192304.1957418-1-Liam.Howlett@oracle.com Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()") Signed-off-by: Liam R. Howlett Suggested-by: Jann Horn Cc: Jason A. Donenfeld Cc: Matthew Wilcox Cc: SeongJae Park Cc: Vlastimil Babka Cc: Yu Zhao Signed-off-by: Andrew Morton --- mm/mmap.c | 11 +++-------- 1 file changed, 3 insertions(+), 8 deletions(-) (limited to 'mm/mmap.c') diff --git a/mm/mmap.c b/mm/mmap.c index 0024e615d069..1d9aac3aa899 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2948,9 +2948,9 @@ static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *vma, * Expand the existing vma if possible; Note that singular lists do not * occur after forking, so the expand will only happen on new VMAs. */ - if (vma && - (!vma->anon_vma || list_is_singular(&vma->anon_vma_chain)) && - ((vma->vm_flags & ~VM_SOFTDIRTY) == flags)) { + if (vma && vma->vm_end == addr && !vma_policy(vma) && + can_vma_merge_after(vma, flags, NULL, NULL, + addr >> PAGE_SHIFT, NULL_VM_UFFD_CTX, NULL)) { mas_set_range(mas, vma->vm_start, addr + len - 1); if (mas_preallocate(mas, vma, GFP_KERNEL)) return -ENOMEM; @@ -3037,11 +3037,6 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags) goto munmap_failed; vma = mas_prev(&mas, 0); - if (!vma || vma->vm_end != addr || vma_policy(vma) || - !can_vma_merge_after(vma, flags, NULL, NULL, - addr >> PAGE_SHIFT, NULL_VM_UFFD_CTX, NULL)) - vma = NULL; - ret = do_brk_flags(&mas, vma, addr, len, flags); populate = ((mm->def_flags & VM_LOCKED) != 0); mmap_write_unlock(mm); -- cgit v1.2.3